This marks the first official confirmation that BP's spill, which began gushing in April, is the source of the undersea oil, USF chemical oceanographer David Hollander said.

The reason he's sure: Chemical fingerprints don't lie. His team was able to match oil droplets found in plumes beneath the ocean to samples provided by BP.

The finding should forever alter the belief that an oil spill is confined to the surface.

"What we have learned completely changes the idea of what an oil spill is," Hollander said. "It has gone from a two-dimensional disaster to a three-dimensional catastrophe."

While the oil that floats atop the waves decays into tar balls, the oil droplets below the surface stay liquid far longer, Hollander said. The droplets registered on the USF instruments at concentrations just short of the level considered poisonous to humans, he said.

"They're just below toxic," he said.

This also means BP and its partners should bear the expense for dealing with any effects on marine life in the areas where the plumes were found, he said. It also helps explain some of the discrepancies between how much oil was spewing from the broken rig versus how much showed up on the surface.

There remain plenty of unknowns, including whether the undersea plumes were the result of BP spraying thousands of gallons of chemical dispersants directly at the gusher a mile below the surface.

Also unknown: the effect on fish, shrimp and other marine life. The microscopic oil droplets could be absorbed by tiny animals at the base of the food chain, eventually contaminating larger creatures.

A USF research ship found the invisible droplets in samples from 10 locations that, when pieced together, showed two separate plumes. One lay 45 nautical miles north-northeast of the Deepwater Horizon site, while the other was 24 nautical miles east of the rig.

One plume stretched for 20 miles, the other for 22. The shorter one was deeper — about 3/4 of a mile below the surface, compared with half a mile deep for the longer plume.

Hollander and other scientists took the samples in May. They located the undersea oil clouds after models created by USF ocean circulation expert Robert Weisberg predicted that underwater currents would push the oil toward the north-northeast.

When Hollander announced the discovery of the plumes June 8, BP officials scoffed. Everyone knew oil floated on the surface, they insisted. At first they would not hand over any samples from the Deepwater Horizon spill so USF could see if they matched.

BP at last relented and delivered a 2-liter sample to Hollander on June 25. Hollander credited state Rep. Kathy Castor, D-Tampa, for pushing BP to give the USF scientists what they needed.

When BP finally handed over the sample, Castor pointed out they had no reason to hold back: "There's plenty of oil."